... synthesis of ring-expanded analogues of xanthine, guanine, and hypoxanthine, bearing the title heterocyclic ring system [1 ... In view of a number of alleged seven-membered ring heterocycles whose struc- tures were ... g (76%), mp 221-223°; 'H nmr (DMSO-d6): S 9.21 (t, J = 4.5 Hz ...